Semiconductor devices, including PN junctions, typically conduct current in the forward direction and block current in the reverse direction. When operating in the forward direction, the PN junction is capable of conducting high current density (measured in terms of amperes per unit area), while maintaining relatively low power dissipation. However, when a high enough voltage is applied in the reverse direction, the PN junction could eventually reach avalanche breakdown, at which point it is no longer able to block currents in the reverse direction and a relative small current may conduct in the reverse direction. This relative small current in the reverse direction may cause catastrophic failure within the PN junction.
One example of such semiconductor devices is a photodiode, which may include PN junctions (or PIN junctions). In the photoconductive mode, the photodiode is maintained at zero to slightly reverse bias (i.e., the cathode is positive with respect to anode) so that the photocurrent may be proportional to the irradiance on the photodiode. Since a photodiode is essentially a PN junction, its current carrying properties follow that of a PN junction as described in the previous paragraph. Therefore, the photodiode may be a robust device during normal operation under forward bias. However, if a reverse bias as high as the avalanche breakdown is applied, the photodiode may be permanently damaged and could operate erroneously.
The electrostatic discharge (ESD) effects commonly known to occur in electrical circuits may cause sudden and unexpected high voltage charges over semiconductor devices in both forward and reverse directions. Thus, commercial semiconductor devices may require certain protection schemes against ESD. Current ESD protection techniques try to operate PN junctions in the forward direction during all possible stress conditions, thereby avoiding any potential breakdown in the reverse direction. However, this approach may not be appropriate for semiconductor devices such as photodiodes situated in a photodiode array. Semiconductor devices such as photodiodes need bi-directional ESD protection.